P20C3 code meaning
Reductant Heater C Control Circuit Low
Powertrain system Generic code P20xx codes Check engine light
What P20C3 means
A low C circuit points to heater C being grounded somewhere or drawing excessive current. GM's 2025 DEF module treats heater 3, its second tank heater, exactly like heaters 1 and 2: a low-band voltage with the output off, or drain voltage and current limits with it on, Type A once 2 s have elapsed. On the 2014 LML Duramax (6.6 L) the glow plug control module reports heater currents above 25 A within about 1 s (Type B). The best-documented case is GM bulletin 22-NA-112: on 2020–2022 Silverado and Sierra with the 3.0 L LM2 or 6.6 L L5P, P20BB and P20C3 could set without any defect, and a recalibration of the DEF control module changed the test. Deutz maps the code to its return line heater (FMI 4). On those trucks software comes first; elsewhere look for a chafed harness.
P20C3 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P20C3
- Malfunction indicator lamp, possibly with no real fault present
- Driver message that speed will be reduced without service
- Unheated tank or line section in frost
Common causes of P20C3
- Most likelyOutdated DEF control module calibration on 2020–2022 GM diesel pickups (22-NA-112)
- PossibleHeater C wire grounded through chafing
- Less likelyShorted heater C element drawing overcurrent
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant line heater c
What each part of P20C3 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 0 | Area: fuel and air metering and auxiliary emission controls. |
| C3 | The exact fault in that area. |
What to do next
- Note P20C3 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P20C3
What does P20C3 mean?
P20C3 means "Reductant Heater C Control Circuit Low". A low C circuit points to heater C being grounded somewhere or drawing excessive current. It is a generic code, so it means the same on every make.
Is it safe to drive with P20C3?
Drive gently and only to a workshop. With P20C3 the car can go into limp mode (reduced power) to protect itself.
Will P20C3 fail an emissions test?
Yes, it can. P20C3 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P20C3 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P20C3 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.